The Gilrain came swiftly down from the mountains as did the other rivers of that region; but as it reached the end of the outlier of Ered Nimrais that separated it from the Celos... it ran into a wide shallow depression. In this it wandered for a while, and formed a small mere at the southern end before it cut through a ridge and went on swiftly again to join the Serni.

Ibid., Part 2, Ch 4, The History of Galadriel and Celeborn: Amroth and Nimrodel

Though Serni was the shorter river its name was continued to the sea after its confluence with Gilrain. Its mouth was blocked with shingles, and at any rate in later times ships approaching Anduin and making for Pelargir went by the eastern side of Tol Falas and took the sea-way passage made by the Númenóreans in the midst of the Delta of Anduin.